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Aug 28, 2014 - USDA-ARS, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, Pennsylvania 19038, United States. •S Supporting ...
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Maximizing the Stability of Pyrolysis Oil/Diesel Fuel Emulsions Jonathan A. Martin, Charles A. Mullen, and Akwasi A. Boateng* USDA-ARS, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, Pennsylvania 19038, United States S Supporting Information *

ABSTRACT: Several emulsions consisting of biomass pyrolysis oil (bio-oil) in diesel fuel were produced and analyzed for stability over time. An ultrasonic probe was used to generate microscopic droplets of bio-oil suspended in diesel fuel, and this emulsion was stabilized using surfactant chemicals. The most stable emulsion was produced using a polyethylene glycol dipolyhydroxystearate (PEG-DPHS) surfactant, with a hydrophilic−lipophilic balance (HLB) number of 4.75 and a 32:8:1 ratio of diesel to bio-oil to surfactant, i.e., 20% utilization of bio-oil. This emulsion consisted of uniformly sized droplets with an average diameter of 0.48 μm, with no observed coalescence of droplets after 1 week. If left undisturbed, these droplets would slowly settle to the bottom of the mixture at a rate of only 2.4 mm/day, but this settling can be eliminated with slight mechanical agitation. This level of stability facilitates utilization of 20 wt % raw bio-oil in diesel as a renewable liquid fuel for spray combustion without the need for costly and energy-intensive upgrading. Additionally, GC/MS analysis was used to investigate the relative concentrations of various bio-oil components in the emulsions. This analysis identified levoglucosan as a bio-oil component that may be responsible for the instability of the emulsions. Experiments with bio-oil produced by catalytic fast pyrolysis over HZSM-5 (CFP oil) revealed that the major components of this oil are directly miscible with diesel fuel without the need for an emulsion.



INTRODUCTION At the present, the vast majority of the world’s liquid fuels are derived from fossil fuels, leading to increasing concentrations of carbon dioxide (CO2) in Earth’s atmosphere. To combat this trend, one possible strategy is to introduce biofuels into the liquid fuel stream. These fuels are derived from plant matter that has absorbed CO2 from the atmosphere via photosynthesis, balancing out the CO2 released into the atmosphere when the fuel is combusted. One such fuel is biomass pyrolysis oil (bio-oil), also known as “py-oil” or “biocrude”. Bio-oil is a dark brown, viscous liquid produced from the thermal decomposition of biomass in an oxygen-free environment (pyrolysis). The pyrolysis process produces not only liquid bio-oil but also syngas, another potential fuel similar to natural gas, and biochar, a black powder that can be used both for fuel and for soil amendment. If bio-oil can be successfully utilized as a fuel, then it has the potential to reduce atmospheric CO2,1 and this possibility has spurred a wealth of worldwide research into pyrolysis in recent years.2 Many possible strategies have been proposed for utilizing bio-oil as a fuel.3 Direct combustion of raw bio-oil has been demonstrated in several applications such as boilers,4,5 gas turbines,6,7 and diesel engines.7−9 However, raw bio-oil has many undesirable properties that damage and hinder the performance of combustion machinery, such as high viscosity, corrosiveness, and chemical instability.10 Therefore, use of raw bio-oil “as-is” in internal combustion (IC) engines has presented serious challenges. One proposed solution to this problem is to mix biooil with a conventional fuel such as ethanol or diesel fuel. Fuel mixtures containing pyrolysis oil and ethanol have shown potential for use in boilers11 and gas turbines,12 but as of yet, no research has been done into their use in IC engines. Also, while mixtures containing bio-oil and diesel have shown potential for use in IC engines,13,14 bio-oil and diesel are immiscible by themselves; thus, their mixtures always require a third This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society

component as a mixing agent. Bio-oil contains mostly oxygenated compounds that will not form a stable mixture with the hydrocarbon compounds found in diesel,10 and a mixture of the two would require constant mechanical agitation to prevent phase separation. To solve this problem, use of surfactant chemicals to produce emulsions of bio-oil in diesel for use as a fuel has been explored by many researchers recently.14−18 In the same light, other researchers have also produced emulsions of bio-oil in biodiesel, a renewable substitute for diesel,19,20 but this study focuses on fossil diesel emulsions. Bio-oil/diesel emulsions show promise of improving the resultant fuel properties, compared to pure bio-oil, and could serve as a route for some quantities of bio-oil to enter the fuel supply with only minor adjustments to combustion systems. The key to producing an emulsion of two immiscible streams like diesel and pyrolysis oil is the use of a surfactant chemical. Surfactant molecules are comprised of two parts: a polar, hydrophilic “head” and a nonpolar, lipophilic “tail”. When droplets of a polar liquid (such as bio-oil) are dispersed in a nonpolar liquid (such as diesel), the hydrophilic heads of the surfactant molecules will adsorb onto the droplet’s surface, while the lipophilic tails of the surfactant molecules will point outward into the nonpolar liquid. The surfactant forms a film that surrounds the droplet, lowering the interfacial tension and reducing the tendency of droplets to coalesce when they come in contact with one another. This type of emulsion is called a waterin-oil or W/O emulsion.21 The opposite situation, in which droplets of a nonpolar liquid are dispersed in an aqueous liquid, is known as an oil-in-water or O/W emulsion. It is important to note that, despite bio-oil being referred to as an “oil”, it contains Received: July 9, 2014 Revised: August 27, 2014

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water and other polar liquids and will actually form the “water” phase of these emulsions, while diesel fuel, which is comprised of nonpolar hydrocarbons, will form the true “oil” phase. One of the main limiting factors in the use of bio-oil/diesel emulsions is the emulsion stability, i.e., the tendency of the emulsion to revert into two separate phases. If a bio-oil/diesel emulsion is unstable, it cannot be stored for long periods of time without separating into a diesel-rich top phase and a bio-oil-rich bottom phase. Since liquid fuels are often stored for long periods

of time between production and end use, the stability of bio-oil/ diesel emulsions must be maximized to allow for their use as a fuel. The stability of these emulsions depends largely on the chemical structure of the surfactant and the relative proportions of bio-oil and surfactant of choice. Multiple published studies have examined the stability of bio-oil/diesel emulsions,15,16,18 but these studies differ in the surfactants used and in their methods of quantifying emulsion stability. Chiaramonti et al. used approximately 100 surfactants, of which the Atlox group of surfactants was found to be most effective, and evaluated stability using the change in emulsion viscosity with time. Ikura et al. used the Hypermer (now renamed Zephrym) group of surfactants and evaluated stability using the visual stratification of phases. Guo et al. used Span and Tween groups of surfactants and evaluated stability using droplet sizes. The work presented herein builds on existing knowledge by comparing all of the above surfactants and stability measurements and determining the ideal mixture of surfactants and diesel that maximizes emulsion stability. The study also analyzes the concentration of different pyrolysis oil components in the emulsion to determine which of these components most contribute to instability. Additionally, we examine the possibility of mixing diesel with pyrolysis oil partially stabilized a priori by catalytic fast pyrolysis (CFP) over HZSM-5 (CFP oil). Compared to raw pyrolysis oil, CFP oil typically contains slightly higher concentrations of aromatic hydrocarbon compounds that are miscible with diesel fuel but is produced in lesser yields and at the cost of the catalyst, among other costs.

Table 1. Concentrations (wt %) of Pyrolysis Oil Compounds as Measured by GC/MS after Char Removal and Acetone Evaporation chemical class

bio-oil

CFP oil

acetic acid acetol furansa cyclopentanonesb phenolsc guaiacolsd levoglucosan BTEXe napthalenesf polycyclicsg

1.58% 1.06% 0.50% 0.62% 0.62% 0.33% 3.66% 0.00% 0.05% 0.02%

0.22% 0.27% 0.18% 0.81% 5.14% 0.12% 0.77% 1.89% 10.77% 1.78%

a

Furfural, 2(5H)-furanone. b2-Methyl-2-cyclopenten-1-one, 2,3-dimethyl-2-cyclopenten-1-one, 3-methyl-1,2-cyclopentandione. cPhenol, o-cresol, p-cresol, m-cresol, 2,4-dimethylphenol, 4-ethyl phenol. d Guaiacol, 2-methoxy-4-methylphenol, vanillin. eBenzene, toluene, ethylbenzene, p-xylene, o-xylene. fNaphthalene, 1-methyl naphthalene, 2-methyl naphthalene. gIndene, biphenyl, fluorene, anthracene.



MATERIALS AND METHODS

Bio-Oil Production and Analysis. The bio-oil and CFP oil used in these experiments were produced from a mixture of hardwood chips derived from logging operations in Maine. The biomass was fed into the bed of a bubbling fluidized bed reactor22 that was maintained at 500 °C with a stream of pure nitrogen entering from below the bed. The fluidizing medium of this reactor was pure silica sand during the production of raw bio-oil. For CFP oil production, the sand bed was replaced with HZSM-5 zeolite (SiO2/Al2O3 = 30) catalyst to facilitate conversion of the pyrolytic vapors to enrich the concentration of hydrocarbon components of the bio-oil such as benzene, toluene, ethylbenzene, and xylenes (BTEX). After char removal by cyclone separation followed by a series of condensers to remove most of the water, the bio-oil and CFP oil were collected using an electrostatic precipitator (ESP). These samples were then analyzed with a Shiamadzu Gas Chromatograph/Mass Spectrometer QP 2010 (GC/MS) to determine the concentration of their organic components. Over 28 compounds in the produced bio-oil were quantified using calibration curves made from authentic samples purchased from Sigma-Aldrich and used as received. The GC column used was a DB-1701 60 m × 0.25 mm, 0.25 μm film thickness. The oven was programmed to hold at 45 °C for 4 min, ramp at 3 °C/min to 280 °C, and hold there for 20 min. The injector temperature was 250 °C, and the injector split ratio was set to 30:1. The flow rate was 1 mL/min of the He carrier gas. The bio-oil

Table 2. Surfactant Building Blocks

Table 3. Surfactants Used in This Study

a

trade name

head type

tail type

HLB #

previously used by

Span 85 Tween 85 Span 60 Brij 72 Zephrym PD 2206a,b Atlox 4912a

sorbitan PEG sorbitan sorbitan PEG PEG PEG

oleic acid oleic acid stearic acid stearyl alcohol DPHS DPHS

1.8 11.0 4.7 4.9 4.0 6.0

Guo et al.16 N/A Chiaramonti et al.15 N/A Ikura et al.18 Chiaramonti et al.15

See Figure 1 for general structure. bFormerly known as Hypermer B246. B

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samples were prepared as 3% solutions in acetone which were filtered through a 0.45 μm PTFE filter prior to injection. The 28 most abundant quantified compounds were classified into 10 groups, as shown in Table 1. Although cyclones remove the majority of the char, their design efficiencies are only rated at about 95% and hence char particles can still

be found in the collected bio-oil. In order to completely remove the char from the bio-oil prior to emulsification, the bio-oil was first diluted 20% by weight with acetone to reduce its viscosity. The diluted bio-oil was filtered three times through 8, 2, and finally 0.5 μm filters. The acetone was then removed by rotary evaporation at 35 °C. Surfactants. Due to the acidic nature of pyrolysis oils, only nonionic surfactants were used to prevent reactions between the bio-oil components and the surfactant. When nonionic surfactants are used, a measurement system called the hydrophilic−lipophilic Balance (HLB) number is commonly used to select an ideal surfactant mixture:23

HLB =

MWhead × 20 MWhead + MWtail

(1)

In eq 1, MWhead represents the molecular weight of the hydrophilic head section and MWtail represents the molecular weight of the lipophilic tail section. To select an ideal surfactant, one must first determine the ideal HLB of the emulsion and then compare different head/tail

Figure 1. General structure of PEG-DPHS surfactantsexact structure is proprietary information of Croda Inc.

Table 4. Comparison of Surfactant Types under a Microscope, 100× Magnification

C

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Table 5. Comparison of HLB Numbers under a Microscope, 4 Days after Emulsification

combinations that produce the ideal HLB. Although one surfactant may not have quite the ideal HLB, it can be mixed with another surfactant to bring the HLB to the desired value. Typically, an HLB of 4−8 is considered best for W/O emulsions such as the bio-oil/diesel fuel emulsions of interest, while an HLB of 10−18 will be best for O/W emulsions.23 Table 2 presents the most common “head” and “tail” types of nonionic surfactants. The main “head” types are sorbitan and polyethylene glycol (PEG), which contain many polar groups, and the main “tail” types are fatty acids and fatty alcohols, which contain long, nonpolar hydrocarbon chains. Table 3 presents the selected surfactants used in this study, along with their head and tail types. The first two surfactants, Span 85 and Tween 85, were chosen simply because they can be mixed to produce a wide range of HLB values (1.8−11.0) so they were mixed in various proportions to obtain an estimate of the emulsion’s HLB. Once the HLB estimate was obtained, three different head/tail combinations were compared: Span 60, Brij 72, and Zephrym PD2206. Figure 1 shows how Zephrym PD2206 (formerly known as Hypermer B206) exhibits an A-B-A copolymer structure in which two poly(12-hydroxystearate) tails

are esterified onto either end of a PEG chain, forming polyethylene glycol dipolyhydroxystearate (PEG-DPHS).24 Surfactants of this type have been found to be effective with pyrolysis oil mixes in previous studies,15,18 and we anticipated that PEG-DPHS surfactants would be most effective for this study. Thus, we chose another PEG-DPHS surfactant with a higher HLB number, Atlox 4912,25 to mix with the Zephrym PD2206 to produce PEG-DPHS surfactants with a range of different HLB numbers. Emulsification Setup. Emulsification was carried out in batches of 10−15 mL in 4 dram glass vials. The bio-oil was added to the bottom of the vials, followed by the surfactant in an amount to produce the desired bio-oil:surfactant ratio. Diesel fuel obtained from a commercial pump was added on top of the surfactant in an amount 4 times the weight of the bio-oil. The vial was held in a 70 °C water bath to soften the bio-oil and surfactant, and a stainless steel ultrasonic probe was inserted, with the end submerged down into the bio-oil but not touching the bottom or sides of the vial. The probe was connected to a Heat Systems Ultrasonics W-385 pulse generator, and a 50 W, 20 kHz pulse was applied for 5 min to emulsify the mixture. Once the emulsification was complete, the vial D

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was removed from the water bath and all subsequent experiments were performed at room temperature. Evaluation of Emulsion Stability. The visible phase separation of the emulsions was monitored by leaving the vials completely undisturbed and measuring the vertical distance to each phase boundary with a ruler. The microscopic structure of the emulsions was photographed via optical microscopy using a Leica TCS SPE microscope. From these photographs, the size of the droplets was measured using the National Institute of Health’s ImageJ software. The viscosity of the emulsions was measured using a Grabner Minivis II rolling-ball viscometer, during which a 0.4 mL sample was drawn from the emulsion vial. These samples were taken from the vertical center of the emulsion, i.e., halfway between the bottom of the vial and the meniscus of the fluid, to capture the portion of the bio-oil that remained suspended in the diesel fuel and had not yet settled to the bottom. These samples were then analyzed in the GC/MS as described above for the raw bio-oil using a 5 wt % solution in acetone. There are two ways in which an emulsion can separate into phases: creaming/settling and coalescence. Creaming/settling occurs when droplets float to the top (creaming) or sink to the bottom (settling) of the mixture, and was detected by fewer observed droplets in the emulsion and a decrease in the emulsion viscosity. Coalescence occurs when multiple droplets collide and form a single, larger droplet, and was detected by an increase in the observed droplet size and an increase in the emulsion viscosity. More information and references on the theory of emulsion stability can be found in the Supporting Information. It is important to note that settling does not permanently destabilize the emulsion, since the droplets can be dispersed back into the emulsion through simple mechanical mixing, whereas coalescence is permanent and cannot be reversed without performing the entire emulsification process all over again. Evaluation of Resultant Fuels. The diesel fuel, bio-oil, and CFP oil and their emulsions were analyzed for selected fuel properties: density, viscosity, total acid number (TAN), high heating value (HHV), water content, and total elemental composition. All measurements were carried out in triplicate. Density was measured with a 1 mL volumetric syringe, weighed on an analytical balance. Viscosity was measured with the Grabner Minivis II viscometer as described above. TAN was measured with a Mettler Toledo T70 instrument using wet ethanol as the titration solvent and 0.1 M KOH in isopropanol as the titrant. HHV was measured with a Leco AC600 bomb calorimeter. The water content of each sample was determined by Karl Fischer titration. Finally, the carbon, hydrogen, and nitrogen contents were determined with a ThermoElectron 1112 series flash elemental analyzer, with oxygen content being determined by difference. These numbers were then converted to a dry basis using the measured water content.

Figure 2. Changes in viscosity with different HLB numbers.

Figure 3. Relative concentration of different bio-oil components in emulsions, measured by GC/MS, 4 days after emulsification.

Figure 4. Phase separation with different HLB numbers, 1 week after emulsification. E

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Table 6. Comparison of Bio-Oil:Surfactant Ratios under a Microscope



RESULTS AND DISCUSSION Selection of Ideal Surfactant. The first set of experiments carried out was to establish a rough determination of the HLB number needed to emulsify bio-oil in diesel. Span 85 and Tween 85 were mixed to produce surfactants with HLBs of 1.8, 3.6, 5.5, 7.5, 9.3, and 11.0, and a bio-oil/diesel emulsion was made with each of these HLB numbers. Although all of these emulsions separated completely in less than 10 min, the emulsion containing the surfactant with an HLB of 5.5 was the slowest to separate. This coincides with the HLB range of 4−6 that is typical for this type of emulsion. The second set of experiments sought to evaluate three different types of surfactants: Span 60 (sorbitan stearate), Brij 72 (PEG stearyl ether), and Zephrym PD2206 (PEG-DPHS). Table 4 compares the emulsions prepared with each of these surfactants, as seen under a microscope. Span 60 and Brij 72 were not very effective, forming many droplets larger than 10 μm that

quickly sank to the bottom. The droplets in the Span 60 emulsion did not coalesce, but this emulsion would require constant mixing to prevent phase separation. Zephrym PD2206 produced vastly smaller droplets in the emulsion, with nearly all of the droplets having a diameter less than 1 μm. However, this emulsion still phase-separated quickly due to the presence of microscopic char solids contained in the bio-oil. This caused the relatively large, dense char particles to drag the bio-oil droplets down to the bottom of the mixture. From these observations, it was evident that PEG-DPHS exhibits the best class of surfactants for preparing bio-oil/diesel emulsions, but the char solids must be completely removed from the bio-oil in order to achieve a stable emulsion. Therefore, for further experiments, char was removed via filtration prior to emulsification with diesel. HLB Number with Best Surfactant. Once the best surfactant class was identified (PEG-DPHS), further experimentation was carried out to determine the effectiveness of these surfactants over a range of HLB numbers. For this, two PEG-DPHS surfactants, F

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Stability with Different Bio-Oil:Surfactant Ratios. To determine the ideal bio-oil:surfactant ratio in the emulsion, four new emulsions were made using the PEG-DPHS surfactant with an HLB of 4.75. While all previous emulsions had a biooil:surfactant ratio of 4:1, each of these new emulsions had a different bio-oil:surfactant ratio: 8:1, 16:1, 32:1, and 64:1. Table 6 shows each of these emulsions as observed under a microscope at both 1 min and 1 day after emulsification. The 8:1 emulsion had evenly sized droplets with a mean diameter of 0.48 μm that did not change over a period of 1 week. The 16:1, 32:1, and 64:1 emulsions all contained an uneven distribution of droplet sizes, with the larger droplets sinking to the bottom of the emulsion in less than 1 day. The observed result indicates that, with a biooil:surfactant ratio of 16:1 or greater, there is not enough surfactant to completely coat the droplets that are initially produced, and rapid coalescence will occur until the total surface area is coated with surfactant. The coalesced droplets will quickly settle to the bottom of the mixture. However, with a bio-oil ratio of 8:1, there will be excess surfactant without rapid coalescence. In Figure 5, we confirm this analysis by plotting the measured

Zephrym PD2206 (HLB of 4) and Atlox 4912 (HLB of 6), were mixed to produce surfactants with HLBs of 4.25, 4.50, 4.75, 5.00, 5.25, 5.50, and 5.75, and emulsions were prepared from each. In Table 5, we compare each of these emulsions as observed under a microscope, 4 days after emulsification. As is seen, surfactants with HLBs of 4.25, 4.50, and 4.75 all produced fine, stable droplets with a mean diameter of 0.56 μm. This value compares to a critical droplet size of 0.37 μm needed to completely stabilize the emulsion based on calculations following eq S4 presented in the Supporting Information. Although the droplet size of the resultant blend was larger than this critical value, it was still small enough to reduce the expected rate of settling to only 2.4 mm/day (see eq S3 in the Supporting Information for calculation of settling velocities). However, with HLBs of 5 and above, the droplets became progressively more prone to coalescence. Figure 2 shows how the viscosity of the emulsions at HLB in the 4.25−4.75 range was completely stable but the emulsions at HLB greater than 5.00 exhibited dramatic increases in viscosity. This occurrence corresponds with the observed coalescence of droplets sufficient to reduce the total surface area (see eq S6 in the Supporting Information) to cause excess surfactant to dissolve into the continuous phase, thereby increasing its viscosity. Thus, an HLB of 4.75 or lower will be required to produce a maximally stable bio-oil/diesel emulsion. Figure 3 shows the quantitative GC/MS results of these emulsions after 4 days of settling. These plots indicate that only very slight differences between HLB numbers exist. The emulsion at an HLB of 4.75 contained the highest combined concentration of bio-oil components, measured at 83% relative to the initial concentration of these components in the mixture, although this was not significantly different from the other HLB numbers. In this emulsion, the bio-oil component most notably absent was acetic acid, which was only measured at 40% of its initial concentration. This might indicate that the acetic acid may have reacted with a component of the diesel or other bio-oil components, or that some may have evaporated during the emulsification at 70 °C. Of the other bio-oil compounds, levoglucosan, an anhydrous sugar, was found at 84% of its initial concentration while all other compounds were found at 93−104% of their initial concentrations. When the bio-oil was mixed with diesel without surfactant, three of the most concentrated components of bio-oil (acetic acid, acetol, and levoglucosan) quickly separated, but the other components did not, with them being found at relative concentrations of 63−83% even 4 days after mixing. The observed result indicates that acetic acid, acetol, and levoglucosan are most responsible for the immiscibility of bio-oil and diesel, necessitating the use of a surfactant to mix the two liquids. Figure 4 shows the observed phase separation in the same emulsions after 1 week of settling, along with the expected level of settling calculated using eq S3 of the Supporting Information and the droplet sizes from Table 5. Although the HLB of 4.25 produced the expected level of settling, the HLBs of 4.50 and greater produced much less settling than was expected. This is possibly attributed to there being excess surfactant in the mixture, an occurrence that will likely raise the viscosity of the continuous phase and concomitantly reduce the settling rate. This discrepancy is exacerbated by the coalescence of the droplets at the HLBs ≥5.00, which causes even more surfactant to dissolve in the continuous phase. Therefore, the next experiment was designed to determine the ideal ratio of bio-oil to surfactant that stabilizes the droplets but does not dilute the mixture with excess surfactant.

Figure 5. Changes in viscosity with different bio-oil:surfactant ratios.

viscosities of these emulsions against the theoretical viscosity calculated with eq S1 (Supporting Information). As the figure shows, the 8:1 emulsion maintains a constant viscosity for 1 week, which is greater than the theoretical viscosity, indicating excess surfactant has dissolved in the continuous phase. The other emulsions have viscosities lower than the theoretical viscosity, indicating a lower volume fraction of bio-oil in the continuous phase that continues to decrease with time. An unexpected result seen in Table 6 is the complete absence of droplets in the 64:1 emulsion sample after 1 day of settling. This was unexpected, since this sample appeared very similar to the 32:1 emulsion, and did not appear to have completely separated. This occurrence may mean that the droplets are simply too small to be seen under the optical microscope, as these images were captured using a laser with a wavelength of 0.488 μm, a minimum theoretical resolution of 0.244 μm. However, 0.244 μm is below the critical diameter of 0.37 μm needed to eliminate settling, so the presence of such droplets would be indicated by a lack of observed settling. G

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Figure 6. Relative concentrations of bio-oil components in emulsions, measured by GC/MS.

distributed across the droplets, and droplets containing more levoglucosan will possess a higher density, and thus a higher settling rate. Even within the droplets themselves, it is possible that a sufficient density gradient may exist between the levoglucosan and the other bio-oil components to cause levoglucosan to sink to the bottom of the droplet. Doing so would likely result in stress concentrated at the bottom of the surfactant film and reduce the collision force necessary for droplets to coalesce. Since the density of the droplet causes it to sink faster than those around it, the most likely collision site would be on the bottom of the droplet, where the surfactant film is weakest. Guo et al. have also recently proposed a similar mechanism as the reason for the instability of bio-oil/diesel emulsions.16 Further evidence for this phenomenon can be seen in the phase separation behavior of these emulsions. Figure 7 shows the phase separation 1 day after emulsification, with the theoretical levels of settling as calculated using eq S3 (Supporting Information) and the droplet sizes measured 1 min after

Figure 6 shows the GC/MS results from these emulsions taken 1 min, 1 h, 1 day, and 1 week after emulsification. These figures highlight how changing the bio-oil:surfactant ratio has a greater effect on the concentration of levoglucosan than it does on any of the other bio-oil components. At 1 week after emulsification, levoglucosan has the highest relative concentration (143%) of any of the bio-oil components in the 8:1 emulsion, but there was 0% levoglucosan found in the 64:1 emulsion. Meanwhile, the total relative concentration of all the other bio-oil components besides levoglucosan was 82% in the 8:1 emulsion and 54% in the 64:1 emulsion, so there was a difference of 143% for levoglucosan but only 28% for all the other components when the biooil:surfactant ratio was changed from 8:1 to 64:1. The observed result can be explained by the high density of levoglucosan (1.69 g/mL) compared with the total bio-oil (1.31 g/mL). This density gradient may be affecting both the settling rate and the coalescence of the droplets. Because of the microscopic size of the droplets, different bio-oil components will be unevenly H

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Figure 7. Phase separation with different bio-oil:surfactant ratios.

in diesel fuel itself. The use of the PEG-DPHS surfactants at HLBs of 4.50−5.25 allowed the phenols, acetol, and levoglucosan in the CFP oil to fully mix with the diesel, but the concentration of these components was low enough that it made little difference in the total aggregated concentration of all CFP oil components. No acetic acid was found in any of these mixtures, but this could be due to the fact that its original concentration was so low that it could not be detected under the GC/MS conditions used when diluted with diesel. The HLBs of 5.50 and 5.75 appeared to produce no emulsion, since the GC/MS results were nearly identical to the control sample. From these results, it appears that emulsions of CFP oil are unnecessary due to the miscibility of most of its components in diesel. While the observed visual results of this experiment indicated that there is a significant portion of CFP oil that is not miscible with diesel, there is a significant bottom phase in each mixture 1 week after emulsification (Figure 9). It is likely that this bottom phase consists mostly of nonvolatile materials likely pyrolytic lignin and/or oligomeric carbohydrates which are not detected by the GC/MS. This indicates that pyrolytic lignin is the main substance forming the internal phase of the CFP oil emulsions. However, pyrolytic lignin is undesirable in liquid fuel, and is the primary cause of many of the poor fuel properties of pure bio-oil and CFP oil such as high viscosity and thermal instability.10 Therefore, like levoglucosan in bio-oil, it would be much more advantageous to simply mix the CFP oil with diesel by keeping the pyrolytic lignin separate prior to its use as combustion fuel.28 Thus, it appears that the emulsification of CFP oil in diesel would not be necessary given its reasonable miscibility. Combustion Ramifications of Select Emulsions. Table 7 shows a comparison of selected fuel properties for some of the fuels prepared in this study. The starting materials of #2 diesel, bio-oil, and CFP oil are shown, as well as the most stable bio-oil/ diesel emulsion and the CFP oil/diesel mixture after the pyrolytic lignin had settled out of the mixture. Also included for the sake of comparison is a mixture of 20% bio-oil and 80% ethanol, similar to the mixtures prepared in a recent study on the combustion of bio-oil/ethanol blends.11 The bio-oil/diesel emulsion has much more desirable fuel properties than pure bio-oil or CFP oil,

emulsification (see Table 6). Also shown in Figure 7 is additional phase separation 1 week after emulsification, with the theoretical levels of settling calculated using the droplets sizes measured 1 day after emulsification. After 1 day, the 8:1 emulsion shows the lowest amount of settling, and an observed settling which roughly matches the calculated levels of settling. After 1 week, however, the observed settling does not match the calculated levels; rather, the 8:1 emulsion settles more quickly than expected and the other emulsions settle more slowly than expected. The likely explanation can be traced to the higher concentration of levoglucosan in the aged 8:1 emulsion, which raises the density of the droplets and increases the settling rate. To account for this, a new bio-oil density was calculated using the relative concentrations shown in Figure 6, and the calculated level of settling was revised. Figure 7 shows that these revised calculations are much closer to the observed levels of settling than originally estimated with assumed equal densities for each emulsion. This confirms that levoglucosan separates much more quickly than the other bio-oil components when there is an insufficient amount of surfactant in the mixture. If levoglucosan and other sugars or anhydrosugars present in bio-oil are that efficient in destabilizing bio-oil/diesel emulsions, then it may be advantageous to extract levoglucosan from bio-oil before the emulsification process to facilitate extra use of bio-oil as fuel. Cost permitting, the extracted levoglucosan could then be hydrolyzed to glucose for fermentation to bioethanol, a process that has been investigated by several researchers.26,27 However, further research would be needed to reevaluate and reclassify the emulsification of bio-oil with levoglucosan extracted to establish their stability and the economics of producing such emulsions. Emulsions of CFP Oil. The same experiment shown in the above section “HLB Number with Best Surfactant” was repeated using bio-oil produced by CFP over HZSM-5 (referred to as CFP oil) instead of thermal-only (regular) bio-oil. Figure 8 plots the GC/MS results from these emulsions 1 week after emulsification, along with a control sample in which no surfactant was used. As the figure shows, most of the CFP oil components are miscible with diesel fuel without the need for a surfactant. This is especially so for the alkyl benzenes and naphthalenes, which are also found I

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the emulsion evenly mixed, perhaps just the natural mechanical shaking in a vehicle’s fuel tank while the engine is running, although this settling would still occur when the vehicle turned off. The CFP oil mixture would not require surfactant, but its production is done in lower yield from biomass, requires the use of catalysts, and is currently limited to a much smaller scale than regular bio-oil.2,29 The bio-oil/ethanol mixture would not require surfactant either but would have a much lower heating value. To illustrate why the decrease in viscosity achieved with the bio-oil/diesel emulsion is so beneficial, we carried out a simulation of the spray behavior of the fuels from Table 7. Equation S7 of the Supporting Information was used to compute the theoretical atomization of these fuels in a Delavan 30609-3 oil burner nozzle, which was used previously to study the combustion of bio-oil/ethanol blends.11 This nozzle mixes air and fuel internally using an adjustable flow of compressed air. Figure 10 presents the ranges of spray droplet sizes that this nozzle can theoretically achieve by adjusting the air flow through the nozzle. The results show that, with pure bio-oil, the nozzle can achieve only a coarse spray with droplet diameters of >600 μm, which will lead to very inefficient, dirty combustion. However, with CFP oil, the nozzle can achieve droplet sizes of just below 50 μm at its maximum airflow setting, which is just within the target range of 30−50 μm. The bio-oil/ethanol blend is well within this range but will still require significantly more air flow than diesel due to its lower heating value, which necessitates a higher flow rate of fuel to produce the same heat output. The bio-oil/diesel emulsion and CFP oil/diesel blend are both very close to pure diesel, and will only need an incremental increase in airflow, making them much more feasible as liquid fuels than pure CFP oil and especially pure bio-oil. Although the emulsions produced in this study were not subject to combustion testing, Figure 10 shows that the spray atomization patterns of these emulsions fall within the zones that support pure diesel flames. However, there have been multiple studies on the combustion of bio-oil/diesel emulsions. Chiaramonti et al. studied single-droplet combustion of bio-oil/ diesel emulsions and found that these emulsions have a much lesser ignition delay than pure bio-oil, which greatly increases bio-oil’s potential for use in diesel engines.30 Short-term engine tests have shown that bio-oil/diesel emulsions exhibit reduced NOx emssions compared to diesel fuel, but these emulsions limit the operational range of the engine’s rotational speed and produce higher smoke emissions.14 Also, standard fuel nozzles are seriously corroded by bio-oil/diesel emulsions, but stainless

Figure 8. Concentration of different CFP bio-oil components in emulsions, measured by GC/MS, 1 week after emulsification.

especially the viscosity, which is more than 10 times lower than CFP oil and more than 100 times lower than regular bio-oil. This is due to the nature of the emulsions’ viscosity (see eq S1 of the Supporting Information). The starting viscosity of an emulsion’s internal phase (bio-oil) has no bearing on the viscosity of the emulsion itself. This makes emulsification especially advantageous for a highly viscous fuel such as bio-oil. The emulsion also has a much lower acid number, higher HHV, lower water content, and lower dry oxygen content than either the bio-oil or CFP oil. The lower acid number is especially advantageous for combustion systems that utilize mild steel, which can be seriously corroded by pure bio-oil.3 Although pure diesel fuel may have a higher heating value by mass, when the heating values are converted to a volumetric basis, the bio-oil/diesel emulsion has almost the exact same value as pure diesel (38.2 and 38.0, respectively) due to the high density of bio-oil. Although some settling will occur with this emulsion, it will only occur at a rate of 2.4 mm/day, which is slow enough that gentle shaking can keep

Figure 9. Phase separation with CFP bio-oil, 1 week after emulsification. J

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Table 7. Comparison of Resultant Fuelsa density (g/mL) viscosity @ 20 °C (cP) TAN (mg of KOH/g) HHV (MJ/kg) HHV (MJ/L) water (wt %) C (dry wt %) H (dry wt %) N (dry wt %) O (dry wt %) O:C molar ratio, dry H:C molar ratio, dry settling rate (mm/day) a

#2 diesel

bio-oil

CFP oil

20% bio-oil/80% diesel emulsion

20% CFP oil/80% diesel mixtureb

20% bio-oil/80% ethanol mixture

0.832 2.94 N/A 45.7 38.0 0.0 86.23 13.77 0.00 0.00 0.00 1.92 0.0

1.312 715 15.8 23.5 30.9 8.93 61.35 6.34 0.24 32.07 0.39 1.24 0.0

1.160 78.3 22.5 28.9 33.5 10.60 75.65 5.20 0.17 18.97 0.19 0.81 0.0

0.928 5.94 3.1 41.2 38.2 1.74 80.93 12.26 0.05 6.77 0.06 1.82 2.4

0.898 3.20 4.5 42.3 37.1 2.12 84.11 12.06 0.03 3.79 0.03 1.72 0.0

0.894 2.39 3.2 28.6 25.0 1.79 53.98 11.77 0.05 34.20 0.48 2.62 0.0

All values are averages of triplicate analysis. bAfter settling and removal of pyrolytic lignin.

settling. This emulsion had fuel properties that were very similar to pure diesel fuel and a vast improvement over raw bio-oil. This is especially true with regard to viscosity; the viscosity of the emulsion was less than 1% that of the original bio-oil, which makes it much more feasible as a partial replacement for pure fossil diesel. GC/MS analysis of these emulsions revealed that levoglucosan is the bio-oil component that is most responsible for its instability, and that the extraction of levoglucosan prior to emulsification may significantly reduce the rate of settling and the amount of surfactant needed to stabilize the emulsion. It was also found that most components of catalytic fast pyrolysis oil (CFP oil) are directly miscible with diesel without the use of a surfactant, and that this mixture also has similar properties to pure fossil diesel.



ASSOCIATED CONTENT

S Supporting Information *

Details on the theory and mathematics referenced in the text and figures. Sections: Emulsion Viscosity, Phase Separation by Settling, Phase Separation by Coalescence, and Liquid Fuel Atomization. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author

Figure 10. Calculated mean droplet sizes from spray nozzle.

*Phone: +1 (215) 233 6493. Fax: +1 (215) 233 6406. E-mail: [email protected]

steel nozzles are not, and the use of stainless steel nozzles is essential for these emulsions to be used as a fuel.13 However, long-term engine testing of these emulsions over hundreds of hours has not been performed, and more studies of the actual combustion of these emulsions are needed.

Notes

Disclosure: Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. The authors declare no competing financial interest.



CONCLUSIONS Emulsions of pyrolysis oil (bio-oil) in diesel fuel were studied for their stability with different types and amounts of surfactants. The most stable emulsion was produced using the surfactant class polyethylene glycol dipolyhydroxystearate (PEG-DPHS), with an HLB number of 4.75 and a mass ratio of 32:8:1 diesel:bio-oil:surfactant. This emulsion contained droplets with a constant average diameter of 0.48 μm, which settled to the bottom of the mixture at a rate of 2.4 mm/day. This settling rate is significant but small enough that the inherent mechanical vibrations in a combustion system may be enough to prevent



ACKNOWLEDGMENTS The authors would like to thank Dr. Daniel Solaiman for the use of the ultrasonic probe, Mr. Joseph Uknalis for assistance with the microscope and droplet size analysis, and Mark Schaffer and Frank Lujaji for assistance with the emulsion preparation and analysis. USDA-NIFA-BRDI grant 2012-10008-20271 is hereby acknowledged. K

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(19) Jiang, X.; Ellis, N. Upgrading bio-oil through emulsification with biodiesel: mixture production. Energy Fuels 2009, 24, 1358−1364. (20) Prakash, R.; Singh, R. K.; Murugan, S. Use of Biodiesel and Bio-oil Emulsions as an Alternative Fuel for Direct Injection Diesel Engine. Waste Biomass Valorization 2013, 4, 475−484. (21) Becher, P. Principles of Emulsion Technology; Chapman & Hall: London, 1955. (22) Boateng, A. A.; Daugaard, D. E.; Goldberg, N. M.; Hicks, K. B. Bench-Scale Fluidized-Bed Pyrolysis of Switchgrass for Bio-Oil Production. Ind. Eng. Chem. Res. 2007, 46, 1891−1897. (23) Americas, I. C. I. The HLB System: A Time-saving Guide to Emulsifier Selection; ICI Americas, Incorporated: Wilmington, DE, 1984. (24) Aston, M. S.; George, M. A.; Sawdon, C. A. Method of carrying out a wellbore operation. WO 2012152889 A1, November 15, 2012. (25) ATLOX 4912 Polymeric Surfactant; Croda International: Snaith, UK, 2008 (http://www.crodacropcare.com/home.aspx?d= content&view=dtl&s=143&r=256&p=1932&prodID=214). (26) Lian, J.; Chen, S.; Zhou, S.; Wang, Z.; O’Fallon, J.; Li, C.-Z.; Garcia-Perez, M. Separation, hydrolysis and fermentation of pyrolytic sugars to produce ethanol and lipids. Bioresour. Technol. 2010, 101, 9688−9699. (27) Bennett, N. M.; Helle, S. S.; Duff, S. J. Extraction and hydrolysis of levoglucosan from pyrolysis oil. Bioresour. Technol. 2009, 100, 6059− 6063. (28) Tang, Z.; Zhang, Y.; Guo, Q. Catalytic Hydrocracking of Pyrolytic Lignin to Liquid Fuel in Supercritical Ethanol. Ind. Eng. Chem. Res. 2010, 49, 2040−2046. (29) Ringer, M.; Putsche, V.; Scahill, J. Large-Scale Pyrolysis Oil Production: A Technology Assessment and Economic Analysis; NREL/TP510-37779; National Renewable Energy Laboratory: Golden, CO, 2006. (30) Chiaramonti, D.; Riccio, G.; Baglioni, P.; Bonini, M.; Milani, S.; Soldaini, I.; Calabria, R.; Massoli, P. Sprays of biomass pyrolysis oil emulsions: modeling and experimental investigation. Preliminary results and modeling. In Proceedings of the 14th European biomass conference & exhibition, Paris, France, October 17-21, 2005.

ABBREVIATIONS BTEX = benzene, toluene, ethylbenzene, and xylenes CFP = catalytic fast pyrolysis DPHS = dipolyhydroxystearate GC/MS = gas chromatograph/mass spectrometer HLB = hydrophilic−lipophilic balance HHV = high heating value PEG = polyethylene glycol



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